The present invention controls the uniformity and magnitude of corona charging of a charge retentive, photoresponsive surface. A scorotron is similar to a corotron, but makes use of an open screen grid as a control electrode, to establish a reference potential, so that when the receiver surface reaches the grid's reference potential, the corona generated electric fields no longer drive ions to the receiver, but rather to the grid. Many factors can contribute to charge nonuniformity across the surface of a photoresponsive member. For example, nonuniformity in the thickness of the photoresponsive layers and edge effects both impact the charging characteristics of a photoresponsive member. Furthermore, nonuniformity in charging characteristics, particularly the charge density and the charge potential, can be exacerbated by the charging device utilized, as well as by aging of the photoresponsive member, where higher charge levels are needed to produce a desired potential on the photoresponsive surface.
As represented by the simplified corotron illustrated in FIG. 1, it is well known to surround corona wire 104, by a grounded shield, 106. Moreover, it is known that the resulting ion current flowing to the surface of photoreceptor 20, represented by I.sub.p, can be represented by the following equation: EQU I.sub.p =I.sub.c -I.sub.e, Eq. 1
where I.sub.c is the ion current emitted from corona wire 104, and I.sub.e is the current flowing through grounded shields 106. Similarly, as illustrated in FIG. 2, the addition of shield bias voltage V.sub.B, and scorotron grid 108, having bias voltage V.sub.G applied thereto, will result in a modified ion current flowing to the photoreceptor surface. The modified photoreceptor ion current, I.sub.p ', is represented as follows: EQU I.sub.p '=I.sub.p -I.sub.g, Eq. 2
where I.sub.g is the ion current which is drained off by the biased scorotron grid. Further derivation of the equations for the specific currents as a function of the applied or bias voltage and geometry are described by R. M. Schaffert in Electrophotography, Focal Press, London (1971), the relevant portions of which are hereby incorporated by reference.
Heretofore, numerous variations of corotron and scorotron charging systems have been developed employing the principles represented in FIGS. 1 and 2, of which the following disclosures may be relevant:
U.S. Pat. No. 2,777,957, Patentee: Walkup, Issued: Jan. 15, 1957.
U.S. Pat. No. 2,965,754, Patentee: Bickmore et al, Issued: Dec. 20, 1960.
U.S. Pat. No. 3,937,960, Patentee: Matsumoto et al., Issued: Feb. 10, 1976.
U.S. Pat. No. 4,112,299, Patentee: Davis, Issued: Sep. 5, 1978.
U.S. Pat. No. 4,456,365, Patentee: Yuasa, Issued: Jun. 26, 1984.
U.S. Pat. No. 4,638,397, Patentee: Foley, Issued: Jan. 20, 1987.
U.S. Pat. No. 5,025,155, Patentee: Hattori, Issued: Jun. 18, 1991.
Xerox Disclosure Journal, Vol. 10, No. 3, May/June 1985.
Xerox Disclosure Journal, Vol. 17, No. 4, July/August 1992.
The relevant portions of the foregoing patents may be briefly summarized as follows:
U.S. Pat. No. 2,777,957 discloses a corona discharge device for electrically charging an insulating layer. A conductive grille is interposed between the ion source, for example, the corona discharge electrode, and the insulating layer, preferably a photoconductive insulating layer. The grille is maintained at a potential below the voltage of the corona discharge electrode and produces a uniform charge potential across the insulating layer.
U.S. Pat. No. 2,965,754 describes a double screen corona device having a pair of corona screens to substantially eliminate charge nonuniformity, referred to as charge streaking. The screens, inserted between the corona element and an insulating layer, are arranged in a parallel fashion overlapping one another so as to diffuse the ions emitted by the corona element before they are deposited on an insulating layer. Both screens may be maintained at slightly different potentials, however, the screen closest to the insulating layer is maintained at a potential between four and ten times the maximum potential to which the insulating layer is to be raised.
U.S. Pat. No. 3,937,960 discloses a charging device for an electrophotographic apparatus having a movable control plate. The control plate, commonly referred to as a shield, is formed of a flexible conductive material. The control plate may be moved relative to a corona producing wire, such that the movement of the plate produces a corresponding variation in the ion flow from the wire.
U.S. Pat. No. 4,112,299 teaches a corona charging device having an elongated wire and a surrounding conductive shield which is segmented in a direction parallel to the wire. Each of the conductive shield segments may be biased at different potentials in order to produce a universal corona generating device which is adaptable to a variety of situations.
U.S. Pat. No. 4,456,365 discloses a corona charging device for uniformly charging an image forming member which includes a corona wire and a conductive shield which partially surrounds the wire. The image forming member is uniformly charged by applying an AC voltage to the corona wire, along with an additional DC bias voltage.
U.S. Pat. No. 4,638,397 describes a scorotron where the wire grid is connected to ground via a plurality of Zener diodes and a variable resistor. The control circuit employed effectively limits the charge potential which is deposited on a photoconductive layer by varying the voltage applied to a control grid as a fraction of the nominal voltage applied to the grid.
U.S. Pat. No. 5,025,155 teaches a corona charging device for charging the surface of a moving member which includes a plurality of corona generating electrodes and a grid electrode located between the moving member and the wire electrodes. Increased surface potential is achieved on the moving member utilizing a plurality of wire electrodes, where the distance between the grid electrode and the moving member is shortest beneath the downstream electrode.
Xerox Disclosure Journal (Vol. 10, No. 3; May/June 1985) teaches, at pp. 139-140, a charging scorotron employing a scorotron grid which is segmented on one end thereof in order to selectively avoid the creation of unused charged areas on an adjacent photoreceptor. The two disclosed segments at the end of the scorotron are switchably connected to a potential source so that in all cases the photoreceptor width corresponding to the image size of the smallest copy sheet is always charged.
Xerox Disclosure Journal (Vol. 17, No. 4; July/August 1992) describes, at pp. 239-240 a corrugated scorotron screen having corrugations which run orthogonal to the process direction of a charge receptor. As noted, the added strength and rigidity provided by the corrugations within the screen help to maintain flatness and rigidity of the screen.
In accordance with the present invention, there is provided a charging apparatus adapted to apply a uniform charge to a charge retentive surface. The scorotron apparatus comprises corona producing means, spaced apart from the charge retentive surface, for emitting a corona ion current and means, responsive to a bias voltage, for locally altering the corona ion current passing between said corona producing means and the charge retentive surface.
In accordance with another aspect of the present invention, there is provided an electrophotographic imaging apparatus for producing a toned image, including a photoconductive member, means for charging a surface of said photoconductive member to produce a uniform charge density across a surface thereof, means for exposing the charged surface of said photoconductive member to record an electrostatic latent image thereon, and means for developing the electrostatic latent image recorded on said photoconductive member with toner to form a toned image thereon. The charging means includes corona producing means, spaced apart from the surface of said photoconductive member, for emitting a corona ion current and means for locally altering the corona ion current passing between said corona producing means and the surface of said photoconductive member.